Four-Point Correction

Four-Point Correction

Four-point correction is a simple way to eliminate trapezoid, parallelogram or projective distortions in images (technical drawings mainly). This procedure is based on the assumption that an image frame and its contents are distorted in the same way. This procedure can be used to correct image geometry if its frame has a trapezium or parallelogram shape, rather than a rectangular.
This operation can be applied to several images at once. For example, correct a “pile” of images placed one over another.
To perform this procedure, specify the desired frame size – its height and width, and the appropriate current position of the frame corner dots on the image. After correction the image is transformed so that the frame corner dots are moved to the rectangular frame corners of the specified size.

To perform four-point correction

1. Select the images to correct by four-point correction. If no image is selected, then this command processes all visible images located on unlocked layers.
2. Run the operation using one of the following methods:
Choose 4-point correction from the Image menu or click the button on the Image toolbar.
The following dialog box is displayed:

3. Press the – Find frame button. If WiseImage is able to find the drawing frame, then you will see a colour polygon over the image close to the raster lines. The frame is shown in the document window. If WiseImage can not find the drawing frame, then specify these points manually.
4. To specify the frame corners manually, press the button and click the frame corners on the image. These points can be specified in an arbitrary order because the program always sorts them so they form the frame without intersections. Watch the rubberline to control, press BACKSPACE to go to previous frame corner if necessary.
5. Type the desired frame size in Width and Height.
6. Select the orientation – Portrait or Landscape.
7. Click OK.
The button, the Paper size list box and Use Internal Page Frame checkbox control the automatic paper size adjustment. Use these controls to adjust the image to fit a standard paper size.
The Paper size contains standard paper formats.
Use Internal Page Frame lets the user use the corresponding drawing frame size that is stored for a particular format (internal page frame size is set in Tools > Preferences > Papers > Modify dialog, in Internal Frame section).

If Internal Frame is not checked in this dialog, then the Use Internal Page Frame checkbox in the 4-point correction dialog is unavailable.
If Internal Frame is checked, then settings in Portrait and Landscape sections are automatically reflected in the Width and Heights fields of 4-point correction dialog.